The invention concerns a servo valve controlled by a bistable solenoid valve and activated by the same fluid--liquid or gas--it controls the flow of. A disk is mounted on a differential piston that travels back and forth in the housing. A compression chamber at one end of the piston communicates with a fluid intake and, by way of a seat facing the disk, with a fluid outlet. The piston has an eccentric control bore extending through it. A control chamber at the other end of the piston communicates with the fluid outlet through a depressurization channel and with the compression chamber through the control bore. The solenoid valve has a chamber and seat and an armature that travels back and forth in a tube. The tube extends through a coil on the housing. A gasket is mounted on the end of the armature facing the solenoid valve's seat. The other end faces a head accommodated in the tube. A yoke surrounds both ends of the coil. The depressurization channel extends through the chamber and seat and can be closed off by the plug.
A valve of this type is known. It is described in German 0S 3 822 830 for example. Many embodiments of bistable solenoid valves for servo controlling are known. They make it possible to operate with the least possible power. Another advantage is direct control at the interface without signal processing. A third is that the coil and armature will not heat up. Bistability can usually be attained with a permanent magnet and a matching spring. Solenoid valves without permanent magnets are also known, however. Their bistability derives from their relatively hard-magnetic materials. The coercive field strength of such materials can be either decreased to zero or augmented for a brief period that depends on the polarity of the coil, and the solenoid valve's armature will be either attracted by the polar surface or unattracted and repelled by a compensating spring. Without a permanent magnet, a solenoid valve cannot act as a trap for any iron-containing particles floating in a hydraulic fluid.